Polygraph

What are control questions?

Control questions establish baseline physiological response. It is important that there is variation in control questions, i.e routine/non-emotional questions, and questions intended to increase arousal slightly by making you feel uncomfortable when answering truthfully. Positive and negative questions are also needed (positive questions get a 'yes' response, negative questions get a 'no' response), so that relevant questions demanding either a positive or negative response can be compared. These physiological responses for these two types of control questions can then be compared to get an accurate baseline measure of arousal.

What are the limitations of the polygraph?

Detection relates to similarity of physiological changes associated with different emotions, e.g guilt, fear, anxiety, thus when an innocent person is in a high state of arousal due to these emotions, they may have been incorrectly interpreted as lying. They may be on medication which can vary their rate of metabolic function (affecting physiological arousal). Self-infliction of pain - during the baseline phase, a person may induce a heightened state of arousal, so the accused may have similar levels of arousal when answering control and relevant questions either from the effect of inflicting pain or through lying. People with low heart rates and breathing rates often only show a small difference between physiological responses to relevant and control questions. Sedatives - produce a low reactivity as they slow down the nervous system. Pathological liars - people who can lie without showing physiological arousal. Fluctuating temperatures (internal due to illness, external due to varying room temperature) can affect readings.

What does the polygraph record?

Heart rate, blood pressure, breathing rate and galvanic skin response.

What does the polygraph measure?

Polygraph measures physiological changes associated with arousal.

What are relevant questions?

Relevant questions pertain to the reason for undergoing the test i.e questions relevant to the crime. Relevant questions assumes an innocent person's physiological response to control questions will be similar to their physiological responses to relevant questions. An accused person who is lying would register greater physiological arousal (as indicated by higher breathing rate, galvanic skin response, heart rate, etc) when asked a relevant question when compared with control questions.

What is the presumption behind the polygraph?

The presumption behind the polygraph is that lying produces heightened physiological arousal i.e increased heart rate, breathing rate etc. It compares a person in a relaxed state v when in response to an emotionally charged question.